Vacuum ultraviolet trasmitting silicon oxyfluoride lithography glass

ABSTRACT

High purity silicon oxyfluoride glass suitable for use as a photomask substrates for photolithography applications in the VUV wavelength region below 190 nm is disclosed with the silicon oxyfluoride glass having a preferred fluorine content&lt;0.5 weight percent. The inventive silicon oxyfluoride glass is transmissive at wavelengths around 157 nm, making it particularly useful as a photomask substrate at the 157 nm wavelength region. The inventive photomask substrate is a “dry,” silicon oxyfluoride glass which exhibits very high transmittance in the vacuum ultraviolet (VUV) wavelength region while maintaining the excellent thermal and physical properties generally associated with high purity fused silica. In addition to containing fluorine and having little or no OH content, the inventive silicon oxyfluoride glass suitable for use as a photomask substrate at 157 nm is also characterized by having less than 1×10 17  molecules/cm 3  of molecular hydrogen and low chlorine levels.

CROSS-REFERENCE TO RELATED APPLICATIONS

The application is continuation-in-part to U.S. application Ser. No.09/799,987, filed Mar. 6, 2001, now U.S. Pat. No. 6,492,072 entitledVacuum Ultra-Violet Transmitting Silicon Oxyfluoride Lithoqraphy Glass,of Lisa A. Moore and Charlene Smith, which is a continuation Ser. No.09/397,573, U.S. Pat. No. 6,242,136 filed Sept. 16, 1999, which claimspriority to U.S. Provisional Serial No. 60/135,270 filed May 21, 1999and U.S. Provisional Serial No. 60/119,805 filed Feb. 12, 1999, all ofwhich benefit of priority is claimed.

FIELD OF THE INVENTION

The present invention relates generally to lithography, and particularlyto optical photolithography glass for use in optical photolithographysystems utilizing vacuum ultraviolet light (VUV) wavelengths below 193nm, preferably below 175 nm, preferably below 164 nm, such as VUVprojection lithography systems utilizing wavelengths in the 157 nmregion.

The invention relates to VUV transmitting glass that is transmissive atwavelengths below 193 nm, in particular, a photomask silicon oxyfluorideglass suitable for use in the Vacuum Ultraviolet (VUV) 157 nm wavelengthregion.

BACKGROUND OF THE INVENTION

U.S. application Ser. No. 60/271,136, filed Feb. 24, 2001, entitledVacuum Ultra-Violet Transmitting Silicon Oxyfluoride Lithography Glass,to Lisa A. Moore and Charlene Smith, to U.S. application Ser. No.09/397,572, filed Sep. 16, 1999, entitled Projection LithographyPhotomasks And Method Of Making, of George Berkey, Lisa A. Moore andMichelle D. Pierson, and U.S. application Ser. No. 09/397,577 filed Sep.16, 1999, entitled Projection Lithography Photomask Blanks, Preforms andMethod of Making, of George Berkey, Lisa A. Moore and Charles C. Yu, arehereby incorporated by reference.

Refractive optics requires materials having high transmittance. Forsemi-conductor applications where smaller and smaller features aredesired at the 248 and 193 nm wavelengths, high purity fused silica hasbeen show to exhibit the required minimum transmittance of 99%/cm orbetter.

Projection optical photolithography systems that utilize the vacuumultraviolet wavelengths of light below 193 nm provide benefits in termsof achieving smaller feature dimensions. Such systems that utilizevacuum ultraviolet wavelengths in the 157 nm wavelength region have thepotential of improving integrated circuits with smaller feature sizes.Current optical lithography systems used by the semiconductor industryin the manufacture of integrated circuits have progressed towardsshorter wavelengths of light, such as the popular 248 nm and 193 nmwavelengths, but the commercial use and adoption of vacuum ultravioletwavelengths below 193 nm, such as 157 nm has been hindered by thetransmission nature of such vacuum ultraviolet wavelengths in the 157 nmregion through optical materials. Such slow progression by thesemiconductor industry of the use of VUV light below 175 nm such as 157nm light has been also due to the lack of economically manufacturablephotomask blanks from optically transmissive materials. For the benefitof vacuum ultraviolet photolithography in the 157 nm region such as theemission spectrum VUV window of a F₂ excimer laser to be utilized in themanufacturing of integrated circuits there is a need for mask blanksthat have beneficial optical properties including good transmissionbelow 164 nm and at 157 nm and that can be manufactured economically.

The present invention overcomes problems in the prior art and provides aeconomical high quality improved photomask blanks and VUV transmittinglithography glass that can be used to improve the manufacturing ofintegrated circuits with vacuum ultraviolet wavelengths.

Use of high purity fused silica as optical elements in photolithographystems from the fact that high purity fused silica is transparent over awide range of wavelengths, spanning from the infrared to deepultraviolet regions. Furthermore, high purity fused silica exhibitsexcellent chemical durability and dimensional stability. Theseproperties have made high purity fused silica highly suited for use asoptical lenses as well as for photomask substrates in photolithography,but use has been limited to the KrF and ArF wavelength regions.

Photomask glass qualifications are comparatively different from otheroptical elements used in photolithography in that theycharacteristically have smaller thicknesses of as low as only a fewmillimeters through the optical path. As such, they must meet verystrict requirements for dimensional stability (warping and shrinkage) inorder to ensure the extreme accuracy required to form fine circuitpatterns on the photomask plate and target. And as the demand for evensmaller features continues to drive the lasing wavelength further downto the 157 nm region and lower, the choice of optical materials meetingthe minimum required transmittance becomes severely limited for alloptical elements, but even more so for photomask substrates due to thereasons stated above. Crystalline materials such as calcium fluoride,barium fluoride and magnesium fluoride for example, have been shown toexhibit transmittances which are suitable for 157 nm wavelengthapplications. Unfortunately, these materials tend to have certaindrawbacks making them unsuitable for these applications, in addition tomanufacturing/economic problems. For example, calcium fluoride exhibitsunacceptably high thermal expansion properties for photomaskapplications in the 157 nm wavelength region. Magnesium fluoride on theother hand, exhibits acceptable expansion but is unsuitable because itis naturally birefringment.

Accordingly, it is an object of the present invention to disclose VUVtransmitting silicon oxyfluoride glasses for use at VUV wavelengthsbelow 193 nm, preferably in the F₂ Excimer Laser 157 nm region, methodsof making such glass, and methods for characterizing such siliconoxyfluoride glass.

SUMMARY OF THE INVENTION

In the present invention we disclose VUV transmitting siliconoxyfluoride lithography glass suitable for use as optical elements, foruse as a lens or preferably for use as a photomask substrate at VUVwavelengths below 193 nm. In particular, the inventive siliconoxyfluoride glass exhibits certain properties tailored for applicationsin the photolithography VUV wavelength region around the 157 nm Excimerlaser wavelengths and below 193 nm.

The object of the invention is achieved by use of a dry low hydroxyradical fluorine-doped SiO₂ fused synthetic silicon oxyfluoride glasswhich exhibits very high transmittance in the vacuum ultraviolet (VUV)wavelength region while exhibiting excellent thermal and physicalproperties. By “dry” we mean having an OH content below 50 ppm byweight, preferably dehydrated below 10 ppm OH by weight, and mostpreferably below 1 ppm by weight.

In another aspect, the object of the invention is further achieved byensuring that the silicon oxyfluoride glass is essentially free ofchlorine.

In yet another aspect, the object of the invention is achieved byensuring a low molecular hydrogen content in the glass. By this we meanthat the molecular hydrogen (H₂)content is below 1×10¹⁷ molecules/cm³.

In a preferred embodiment of the invention, the VUV transmitting siliconoxyfluoride glass has a fluorine content in the range of 0.1 to 0.4weight percent which inhibits laser exposure induced absorption andprovides laser exposure durability with minimal transmission loss at157.6 nm after prolonged exposure. The invention includes a below 193 nmVUV transmitting glass photomask substrate for photolithography atwavelengths of about 157 run with the glass being a high purity siliconoxyfluoride glass with an OH content below 50 ppm by weight, hydrogencontent below 1×10¹⁷ molecules/cm³ and a fluorine content in the 0.1 to0.4 weight percent range. The invention includes a process of making VUVtransmitting glass silicon oxyfluoride glass that includes providingparticles of SiO₂, dehydrating the particles, and fluorine doping andconsolidating the particles to form a dry, non-porous monolithic body oftransparent fused silicon oxyfluoride glass with a fluorine content lessthan 0.5 weight percent. The invention includes a silicon oxyfluorideglass having essentially no OH groups, less than 5×10¹⁶ molecules/cm³ ofmolecular hydrogen, and a fluorine content in the range of 0.1 to 0.4weight %. The invention includes a silicon oxyfluoride lithography glasshaving an OH content less than 5 ppm by weight, a Cl content less than 5ppm by weight, a H2 content less than 1×10¹⁷ molecules/cm³, and afluorine content of 0.1 to 0.4 weight % with a 157 nm internaltransmission of at least 85%/cm. The invention includes a VUV patternprinting method with the steps of providing a below 164 nm radiationsource for producing VUV photons, providing a silicon oxyfluoride glasshaving less than 5 ppm by weight OH, less than 5 ppm by weight Cl, a<0.5weight percent fluorine content, and 157 nm and 165 nm measuredtransmission of at least 75%/5 mm. The pattern printing method includestransmitting the VUV photons through the silicon oxyfluoride glass,forming a pattern with the VUV photons and projecting the pattern onto aVUV radiation sensitive printing pattern. The invention includes a VUVtransmitting silicon oxyfluoride glass having a OH content less than 5ppm by weight, a fluorine content of at least 0.1 weight %, the glassconsisting essentially of Si, O, and F with an internal transmission inthe wavelength range of 157 nm to 175 nm of at least 85%/cm and a 165 nmabsorption less than 0.4 (absorption units/5 mm) after exposure to a 157nm laser for 41.5 million pulses at 2 mJ/cm²-pulse.

Additional features and advantages of the invention will be set forth inthe detailed description which follows, and in part will be readilyapparent to those skilled in the art from that description or recognizedby practicing the invention as described herein, including the detaileddescription which follows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are merely exemplary of theinvention, and are intended to provide an overview or framework forunderstanding the nature and character of the invention as it isclaimed. The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate various embodimentsof the invention, and together with the description serve to explain theprincipals and operation of the invention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a VUV spectra comparison plot of % transmission vs. wavelength(nm) with glass C containing 0.8 wt. % F in accordance with theinvention.

FIG. 2 is an optical density plot of optical density vs. pathlengthsample thickness for 0.8 wt. % F glass C (mm) in accordance with theinvention.

FIG. 3 is a before/after F₂ laser radiation exposure absorption spectralplot of optical density vs. wavelength (nm) for 1.1 mm thick sampleswith glass B having 0% F and glass D having 0.2 wt. % F in accordancewith the invention.

FIG. 4 is a refractive index as a function of wavelength (nm) plot forglass E (0.8 wt. % F) showing 3-term Sellmeier fit and extrapolation to157 nm in accordance with the invention.

FIG. 5 is a 435.8 nm refractive index as a function of fluorine content(wt. % F) in accordance with the invention.

FIG. 6 is a 157 nm refractive index (calculated from Sellmeier fit) as afunction of fluorine content (wt. % F) plot in accordance with theinvention.

FIG. 7 is a coefficient of thermal expansion (CTE, 300° C.-RT (ppm/°C.)) as a function of fluorine content (wt. % F) in accordance with theinvention.

FIG. 8 shows a method/lithography system in accordance with theinvention.

FIG. 9 shows a method/lithography system in accordance with theinvention.

FIG. 10 shows a method /lithography system in accordance with theinvention.

FIG. 11 is a plot of 157 nm absorption of 5 mm thick samples vs.chlorine concentration (Cl wt. %) in accordance with the invention.

FIG. 12 is a plot of induced 157 nm absorption of 5 mm thick samples vs.number of pulses (MM) of a F₂ Excimer Laser (2mJ/cm²-pulse) inaccordance with the invention.

FIG. 13A is an absorption spectra before and after F₂ Excimer LaserExposure (2 mJ/cm²-pulse) (41.5 million pulses) of a high fluorinesample (<0.5 ppm OH by weight) with 1.7 wt. % F.

FIG. 13B is an absorption spectra before and after F₂ Excimer LaserExposure (2 mJ/cm²-pulse) (41.5 million pulses) of a low fluorine sample(<0.5 ppm OH by weight) with 0.12 wt. % F.

FIG. 13C is an absorption spectra before and after F₂ Excimer LaserExposure (4.5 mJ/cm²-pulse) (0.96 million pulses) of a no fluorine dry(<0.5 ppm OH by weight) sample with 0 wt. % F.

FIG. 14A is an absorption spectra before and after ArF Excimer LaserExposure (9 mJ/cm²-pulse) (153 million pulses) of a high fluorine sample(<0.5 ppm OH by weight) with 1.7 wt. % F.

FIG. 14B is an absorption spectra before and after ArF Excimer LaserExposure (9 mJ/cm²-pulse) (158 million pulses) of a low fluorine sample(<0.5 ppm OH by weight) with 0.12 wt. % F.

FIG. 14C is an absorption spectra before and after ArF Excimer LaserExposure (25 mJ/cm²-pulse) (74 million pulses) of a no fluorine dry(<0.5 ppm OH by weight) sample with 0 wt. % F.

FIG. 15 is an online 157 nm transmission comparison as a function ofexposure time with the Y-axis % transmission at 157.6 nm/6.35 mm vsX-axis Number of Pulses (millions) of F₂ Excimer Laser (0.2 mJ/cm² perpulse) of a high fluorine sample (<0.5 ppm OH by weight) with 1.7 wt. %F and a low fluorine sample (<0.5 ppm OH by weight) with 0.12 wt. % F.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings.

We have shown that high transmission at below 190 nm, in particularbelow 175 run, and most preferably at the F₂ Excimer Laser wavelengthoutput centered around 157 nm can be provided in SiO₂ containing siliconoxyfluoride glasses by minimizing the OH or water content of the glass.Specifically, we have demonstrated that low OH silicon low chlorineoxyfluoride glass exhibits high transmissivity while having beneficialchemical, physical and mechanical.

The transmission properties of glass in general are dependent on glasscomposition. In pure silica, it has been demonstrated that even tracelevels (ppm or less) of metal contaminants can cause significantreductions in transmission in the ultraviolet region. With this as abackdrop, we have demonstrated that besides metal impurities, the mostimportant variables for controlling the VUV transmission edge of siliconoxyfluoride glass include its water or OH content, as well as itschlorine content. Specifically, we have found that the lower the OHcontent, the better the transmission, while the higher the chlorinecontent, the lower the transmission in the VUV 157 nm region. Inaddition, we have found that the amount of molecular hydrogen in theglass has to be minimized. Most preferably the SiO₂ containing siliconoxyfluoride glass has at least 0.5 wt. % Fluorine. In an alternativelypreferred embodiment the silicon oxyfluoride glass has a fluorinecontent in the range of 0.1 to 0.4 weight percent.

In a preferred embodiment measurements of the infrared transmission ofglass at 2.7 microns is used to quantify the OH content of glass.

In the following table, we compare the physical and mechanicalproperties of the inventive low OH silicon oxyfluoride glass with thoseof a comparative fused silica high OH material.

The inventive silicon oxyfluoride glass may be manufactured byadaptation of a variety of methods such as by hydrolysis/pyrolysis(flame hydrolysis), thermal decomposition (soot process), and oxidationof silicon-containing compounds. Other methods include sol gelprocesses.

A. Silica-forming Processes

Soot Deposition or CVD Process: Typically, in this process thesilicon-containing compound is heated to a constant temperature at whichenough vapor pressure is generated to produce a reasonable rate ofdeposition. The vapors are entrained in a carrier gas stream and passedthrough a burner flame (e.g., natural gas/oxygen mixture,hydrogen/oxygen mixture) to convert the vapor into silica upon exitingthe burner and to form a stream of volatile gases and finely-divided,amorphous, spherical aggregates (soot). In the Outside Vapor Deposition(OVD) variation of this process, the soot is collected on a mandrel thattraverses through the flame to build up a porous silica soot pre-formbody. In the Vapor Axial Deposition (VAD) variation, a porous sootpre-form body is built up by depositing soot on the end of a mandrelpreferably with the deposition process vertically oriented to form avertically oriented soot log. In either case, the porous silica pre-formis subjected to a subsequent high temperature treatment to consolidatethe amorphous particles into a non-porous monolithic glassy fused silicabody. In order to adapt the soot or CVD process to the presentinvention, in addition to consolidation, the amorphous SiO₂ sootparticles are exposed to a fluorine atmosphere fluorine doping agent asmore fully described below. Most preferably in addition to such fluorinedoping of the SiO₂, the SiO₂ is dehydrated in order to eliminate OHgroups associated with the SiO₂.

Dry, fluorine-doped silica can also be formed by casting high puritysilica powder to form a porous silica preform, exposing the body to afluorinating gas, and sintering the porous preform to form a monolithicfused silica body. High purity silica powders can be made by either avapor phase method or by a sol gel method. The advantage of this methodis that the soot can be cast and sintered into a desired shape, such asa plate. In a preferred embodiment the silica powder is a siliconoxyfluoride silica powder. Such casting of a silicon oxyfluoride silicapowder can reduce the degree of exposure of the body to a fluorinatinggas or may be used to replace the need to expose with fluorinating gas.

To ensure that the glass resulting from the above methods is essentiallychlorine-free, the silica-containing starting material is preferablychlorine-free. Useful starting materials include silanes and siloxanes,in particular polymethylsiloanes such as polymethylcyclosiloxane, andhexamethyldisiloxane. Useful polymethylcyclosiloxanes includeoctamethylcyclotetrasiloxane, decamethylcyclopentasiloxane,hexamethylcylotrisiloxane and mixtures of these. Another usefulchlorine-free feedstock for making chlorine-free fused silica includesmethyltrimethoxysilane. If chlorine containing feedstocks and/orchlorine gas agents are utilized, subsequent production processes shouldbe controlled to minimize chlorine in the produced glass and subsequentprocess steps such as fluorine doping with fluorine atomospheres shouldbe controlled and include sufficient chlorine removing agents such assufficient levels of fluorine doping agents and helium, and sufficientreaction times/dynamics to eliminate chlorine.

Since fluorine is a desirable component of the inventive fused silica,fluorine-containing starting materials such as silicon fluoride, silicontetrafluoride and mixtures of these, may be used in the above methods.In addition to utilizing silicon fluoride starting materials, fluorinesource starting materials such as CF₄ and C₂F₆ can be used along withsilicon-containing starting material compounds in combustion/oxidationprocess reacting to produce fluorine doped silica such as siliconfluoride silica soots and powders.

B. Dehydration and Fluorine Doping Process

In order to form dry SiO₂ (i.e., SiO₂ having little or no OH groups)according to the present invention, the pre-consolidated pre-form bodiesformed by the above methods are exposed to a dehydrating gas agentpreferably a halogen-containing atmosphere at elevated temperatures,then sintered to a monolithic fused silica body at even highertemperatures. In an embodiment the dehydration process involves heatingthe porous silica pre-form in a He/dehydrating gas agent atmosphere(such as helium/halide, helium/Cl₂) at a temperature between 1000 and1100° C., to remove water from the silica soot, then sintering thepre-form by passing it through a high temperature zone (typically1450-1500° C.) to form a non-porous, monolithic dry fused silica body.To produce a fluorine-doped glass, preferably, after the dehydrationstep the soot pre-form is exposed to an atmosphere of He and afluorine-containing gas, such as CF₄, SiF₄, SF₆, F₂, C₂F₆, C₃F₈ andmixtures of these, at temperatures in the range of 1150 to 1250° C. Eventhough the fluorine doping step can be done either before, during orafter the dehydration step, it is preferable to do the fluorine dopingafter a chlorine drying step because this allows the fluorine todisplace any chlorine which may be present in the glass, and provide aglass in which chlorine has been eliminated. Also, since the fluorinedoping is typically done at a higher temperature than the dehydrationstep there is a tendency for the glass to sinter and “trap” thefluorine. Further, if the fluorine doping is done before or during thedehydration step some of the fluorine will tend to outgas from the blankat the lower drying temperature. In such a process the blank will alsocontain more residual and detrimental chlorine. Finally, by fluorinatingafter the dehydration step there is an added benefit since thefluorinating gas itself tends to further dry the blank. Additionallyfluorine levels can be improved by utilizing silicon oxyfluoride silicaparticles in the making of the pourous preform. Such silicon oxyfluoridesilica particles are preferably achieved by combusting the siliconfeedstock (such as SiCl₄ or OMCTS) along with a fluorine source (such asCF₄ or C₂F₆). Additionally the fluorine doping process can be improvedby retaining a large level of flourine in the porous preform prior toand during consolidation/sintering into a non-porous body. Suchretention can be improved by forming a non-porous exterior layer on theporous preform body (such as high temperature radiation thermaltreatment focussed on the exterior surface) and inputting the fluorinecontaining gas into the interior of the porous preform such that thenon-porous exterior layer forms an exterior shell barrier forcontrolling the escape of fluorine from the porous preform body. In anembodiment the exterior surface of the porous preform body is sinteredto form a non-porous shell and fluorine is inputted into the porousbody.

While we have described the preferred process as involving both a dryingand a fluorinating step, in an embodiment of the invention we have foundthat high transmittance fused silica suitable for below 193-nmwavelength can also be made by fluorine doping with no prior dryingstep.

The amount of fluorine incorporated in the glass ([F]) is controlled bythe partial pressure of the fluorine-containing gas (P) and thetemperature (T) according to the relationship:

[F]=C×e ^(−E/RT) ×P ^(1/4)

where C is a constant, R is the gas constant, and E is the activationenergy for the reaction between the fluorine-containing gas and silica,for example:

CF₄+4SiO₂=4SiO_(1.5)F+CO₂,

 SiF₄+3SiO₂=4SiO_(1.5)F.

The dry fluorine doped soot pre-form is then sintered by passing itthrough a high temperature zone (typically 1450-1500° C.) to form anon-porous, monolithic dry silicon oxyfluoride SiO₂ glass body asdescribed earlier. The atmosphere around the pre-form during thesintering step may be He or a He/fluorine-containing gas mixture, butpreferably does not contain chlorine in order to minimize incorporationof chlorine into the SiO₂ glass. The amount of fluorine in the fusedsilica is preferably not below 1000 ppm, more preferably 2000 ppm orgreater. In a preferred embodiment fluorine content is in the range of0.1 to 0.4 weight percent.

In addition to being preferably free of OH groups, being fluorine-dopedand essentially free of chlorine, we have found that the inventive fusedsilica photomask blank is also preferably low in molecular hydrogen,preferably less than 1×10¹⁷ molecules/cm³, and more preferably less than5×10¹⁶ molecules/cm³ of molecular hydrogen.

A low OH (OH content<1 ppm) modified fused SiO₂ silicon oxyfluorideglass photomask substrate was doped with 0.8 wt % fluorine and had alowered thermal expansion of 0.52 ppm/° C. and an internal transmittanceof about 84%/cm at 157 nm.

Reducing the OH content to less than 50 ppm, preferably less than 10ppm, and most preferably less than 1 ppm and doping the silica glasswith fluorine provides for increased transmission at 157 nm and inlowered thermal expansion. The preferred 157 nm transmissionphotolithography fluorine doped low OH fused SiO₂ glass photomasksubstrate has a transmittance of at least 80%, preferably at least about83%/cm at 157 nm, and a thermal expansion less than 0.55 ppm/° C.,preferably less than 0.53 ppm/° C.

The photomask substrate was made by forming a silica soot preform. Thesilica soot preform was formed by depositing silica soot produced byconverting silicon tetrachloride (silica feedstock) into SiO₂. Siloxanesilica feedstocks, preferably cyclic siloxanes, and most preferablyoctamethylcyclotetrasiloxane can be converted into SiO₂. Preferably suchconversion of silica feedstocks is achieved by passing the feedstockthrough the conversion site flame of a conversion site burner.

The invention further includes a below 175 nm VUV lithography glass. Thelithography glass comprises a fused silicon oxyfluoride glass. Thesilicon oxyfluoride glass has an OH content less than 5 ppm by weight, aCl content less than 5 ppm by weight, a H₂ content less than 1×10¹⁷molecules/cm³, and a fluorine content of at least 0.1% weight %, withsaid glass having a 157 nm internal transmission of at least 80%/cm andpreferably at least 85%/cm. The silicon oxyfluoride glass has a belowfused silica coefficient of thermal expansion that is less than 0.55ppm/° C. in the room temperature to 300° C. range. Preferably thelithography glass has an internal transmission in the wavelength rangeof 157 nm to 175 nm of at least 80%/cm, and more preferably at least85%/cm. Preferably the silicon oxyfluoride lithography glass has anincrease of absorption at 215 nm of less than 0.1 optical density (log₁₀transmission) per mm when exposed to at least 0.96×10⁶ pulses of 157 nmwavelength containing F₂ excimer laser radiation at 4 m J/cm²-pulse, andmore preferably the increase of absorption at 215 nm is less than 0.05optical density, and most preferably substantially no 215 nm absorptionband is formed. Preferably the Cl content of the glass is less than 1ppm and the OH content is less than 1 ppm, and more preferably the glassconsists essentially of Si, O, and F. Preferably the glass isessentially free of metal to metal Si—Si bonds, and the glass is free ofa 165 nm absorbing center with an internal transmission at 165 nm of atleast 85%/cm.

In a preferred embodiment the lithography glass is used to make a VUVtransmitting photomask where VUV light is transmitted through thephotomask, preferably with a surface of the lithography glass having apatterned deposited film (such as Cr) that forms a transmittingphotolithography mask pattern (FIG. 8). In a further embodiment thelithography glass is used to make a VUV phase shifting photomask wherethe phase of VUV lithography light traveling through the glass isshifted and manipulated to form constructive and/or destructiveinterference patterns (FIG. 9). In a further embodiment the lithographyglass with a lowered thermal expansion and a thermal expansioncoefficient less than 0.55 ppm/° C. is used to make a reflectivephotomask wherein a reflective patterned lithography mask pattern issupported by said silicon oxyfluoride glass (FIG. 10).

The invention further includes a pattern printing method of VUVlithography which includes providing a below 164 nm radiation source forproducing VUV lithography photons, providing a silicon oxyfluoridelithography glass having less than 5 ppm by weight OH, less than 5 ppmby weight Cl, and 157 nm and 165 nm measured transmission of at least75%/5 mm, transmitting the VUV lithography photons through the providedsilicon oxyfluoride lithography glass, forming a lithography patternwith the photons, and reducing the formed lithography pattern andprojecting the formed pattern onto a VUV radiation sensitive lithographyprinting medium to form a printed lithography pattern (FIG. 8).Providing the oxyfluoride lithography glass preferably includes loweringthe VUV cutoff wavelength of the glass by providing an SiO₂ glassforming precursor, lowering the H₂, the OH, and the Cl content of theglass precursor and increasing the F content of the glass precursor toprovide a silicon oxyfluoride glass with a 50% transmission VUV cutoffwavelength below 160 nm. Preferably the provided glass consistsessentially of Si, O, and F and is essentially free of Si—Si bonds.

The invention as shown in FIG. 8 provides a 157 nm photolithographyphotomask substrate photomask stage and a 157 nm photolithography device(157 nm illumination system, photomask—mask stage, 157 nm projectionoptics system, 157 nm wafer stage) with such a fluorine doped low OHsilicon oxyfluoride photomask silica glass substrate that has an OHcontent less than 1 ppm, a fluorine content in the range from 0.1 to 1.5wt. %, 157 nm internal transmittances of at least 50%, and preferably atleast 65%, and most preferably at least 83%/cm at 157 nm, and preferablya thermal expansion less than 0.55 ppm, preferably less than 0.53 ppm,and most preferably less than or equal to 0.52 ppm/° C.

EXAMPLE 1

A 1209 gram, 70 cm long porous silica preform was made by the OVD methodusing SiCl₄ as the feedstock. The soot preform was dehydrated in afurnace at 1000 C. for 60 minutes in an atmosphere of 0.066 slpm Cl₂ and40.64 slpm He. The atmosphere was changed to 40 slpm He and the furnacetemperature ramped to 1100 C. over 20 minutes. The atmosphere waschanged to 0.4 slpm CF₄ and 40 slpm He and the soot preform was held at1100 C. for 120 minutes. The atmosphere was then changed to 40 slpm Heand the soot preform was translated at a rate of 0.5 cm/min into thebottom zone of the furnace which was held at 1480 C. in order to sinterit to a fully dense glassy body. A 5 mm thick sample was cut from theglass preform and polished. The average F concentration of the samplewas determined by microprobe analysis to be 0.17 wt. % (1700 ppm wt.).The average Cl concentration was determined by microprobe analysis to be0.0011 wt. % (11 ppm wt.). The OH content was measured by FTIR method tobe below detection limit (<1 ppm). A transmission of 67.2%/5 mm at 157nm was measured using a vacuum UV spectrophotometer.

EXAMPLE 2

A 1004 gram, 70 cm long porous silica preform was made by the OVD methodusing SiCl₄ as the feedstock. The soot preform was dehydrated in afurnace at 1000 C. for 60 minutes in an atmosphere of 0.066 slpm Cl₂ and40.64 slpm He. The atmosphere was changed to 40 slpm He and the furnacetemperature ramped to 1225 C. over 45 minutes. The atmosphere waschanged to 0.8 slpm CF₄ and 39.2 slpm He and the soot preform was heldat 1225 C. for 120 minutes. The soot preform was then translated at arate of 0.5 cm/min into the bottom zone of the furnace which was held at1480 C. under the same atmosphere in order to sinter it to a fully denseglassy body. A 5 mm thick sample was cut from the glass preform andpolished. The average F concentration of the sample was determined bymicroprobe analysis to be 0.80 wt. % (8000 ppm wt.). The average Clconcentration was determined by microprobe analysis to be 0.0010 wt. %(10 ppm wt.). The OH content was measured by FTIR method to be belowdetection limit (<1 ppm). A transmission of 76.8%/5 mm at 157 nm wasmeasured using a vacuum UV spectrophotometer.

EXAMPLE 3

A 1016 gram, 70 cm long porous silica preform was made by the OVD methodusing SiCl₄ as the feedstock. The soot preform was dehydrated in afurnace at 1000 C. for 60 minutes in an atmosphere of 0.066 slpm Cl₂ and20.64 slpm He. The atmosphere was changed to 16 slpm He and the furnacetemperature ramped to 1225 C. over 45 minutes. The atmosphere waschanged to 4 slpm CF₄ and 12 slpm He and the soot preform was held at1225 C. for 180 minutes. The soot preform was then translated at a rateof 0.5 cm/min into the bottom zone of the furnace which was held at 1480C. under the same atmosphere in order to sinter it to a fully denseglassy body. A 5 mm thick sample was cut from the glass preform andpolished. The average F concentration of the sample was determined bymicroprobe analysis to be 1.48 wt. % (14800 ppm wt.). The average Clconcentration was determined by microprobe analysis to be 0.0020 wt. %(20 ppm wt.). The OH content was measured by FTIR method to be belowdetection limit (<1 ppm). A transmission of 73.5%/5 mm at 157 nm wasmeasured using a vacuum UV spectrophotometer.

EXAMPLE 4

A 1000 gram, 50 cm long porous silica preform was made by the OVD methodusing octamethylcyclotetrasiloxane as the feedstock. The soot preformwas placed in a furnace at 1000 C. in an atmosphere of 40 slpm He. Thefurnace temperature was ramped to 1225 C. over 45 minutes. Theatmosphere was then changed to 0.8 slpm CF₄ and 39.2 slpm He and thesoot preform was held at 1225 C. for 120 minutes. The soot preform wasthen translated at a rate of 0.5 cm/min into the bottom zone of thefurnace which was held at 1480 C. under the same atmosphere in order tosinter it to a fully dense glassy body. A 5 mm thick sample was cut fromthe glass preform and polished. The average F concentration of thesample was determined by microprobe analysis to be 0.96 wt. % (9600 ppmwt.). The average Cl concentration was determined by microprobe analysisto be <0.0010 (<10 ppm wt.). The OH content was measured by FTIR methodto be below detection limit (<1 ppm). A transmission of 76.8%/5 mm at157 nm was measured using a vacuum UV spectrophotometer.

EXAMPLE 5

A 3129 gram, 50 cm long porous silica preform was made by the OVD methodusing SiCl₄ as the feedstock. The soot preform was dehydrated in afurnace at 1100 C. for 120 minutes in an atmosphere of 0.4 slpm Cl₂ and40 slpm He. The atmosphere was changed to 40 slpm He and the furnacetemperature ramped to 1150 C. over 20 minutes. The atmosphere waschanged to 2 slpm O2 and 20 slpm He and the temperature increased to1200 C. over an additional 20 minutes. The atmosphere was changed to 1.2slpm SiF₄, 18.8 slpm He, and 0.2 slpm O2 and the soot preform was heldat 1200 C. for 180 minutes. The soot preform was then translated at arate of 0.5 cm/min into the bottom zone of the furnace which was held at1480 C. under the same atmosphere in order to sinter it to a fully denseglassy body. A 5 mm thick sample was cut from the glass preform andpolished. The average F concentration of the sample was determined bymicroprobe analysis to be 1.29 wt. % (12900 ppm wt.). The average Clconcentration was determined by microprobe analysis to be <0.0010 wt. %(<10 ppm wt.). The OH content was measured by FTIR method to be belowdetection limit (<1 ppm). A transmission of 74.9%/5 mm at 157 nm wasmeasured using a vacuum UV spectrophotometer.

Comparative Example 1

A sample of commercial Corning HPFS® brand UV excimer grade high-purityfused silica (Corning, Incorporated, Corning, N.Y., 14831) was obtained.The glass contained no fluorine, no chlorine, 800 ppm OH, and an H₂content>1×10¹⁷ molecules/cm³. A 5 mm thick sample had no transmission at157 nm.

Comparative Example 2

A 2788 gram, 70 cm long porous silica preform was made by the OVD methodusing SiCl₄ as the feedstock. The soot preform was dehydrated in afurnace at 1000 C. for 50 minutes in an atmosphere of 0.066 slpm Cl₂ and40.64 slpm He. The soot preform was then translated at a rate of 0.5cm/min into the bottom zone of the furnace which was held at 1480 C.under the same atmosphere in order to sinter it to a fully dense glassybody. A 5 mm thick sample was cut from the glass preform and polished.The sample contained no fluorine. The average Cl concentration wasdetermined by microprobe analysis to be 0.176 wt. % (1760 ppm wt.). TheOH content was measured by FTIR method to be below detection limit (<1ppm). A transmission of 21.2%/5 mm at 157 nm was measured using a vacuumUV spectrophotometer.

In a further example a SiO₂ soot preform was dehydrated (—OH removed)using a drying treatment atmosphere containing chlorine. Preferably thedrying treatment atmosphere includes Helium in addition to the chlorinesource molecules. Drying treatment atmospheres containing halides can beused to dehydrate the soot and remove —OH. Drying treatment atmospherescontaining fluorine and/or bromine and/or other halides can be used,preferably with helium.

The SiO₂ soot preform was fluorine doped using a doping treatmentatmosphere of CF₄ and helium. Silicon fluoride can also be used as afluorine source in doping the silica with fluorine. Preferably fluorinedoping is done after dehydrating the soot.

The SiO₂ soot preform was consolidated in a sintering treatmentatmosphere of the CF₄ and helium. Silicon fluoride (SiF₄) can also beused as a fluorine source along with helium as the consolidatingatmosphere.

A photomask substrate was formed from the resulting silicon oxyfluorideglass by cutting the glass, polishing the glass and exposing to 157 nmlaser radiation.

The invention includes silicon oxyfluoride modified fused silica glasseswith low OH contents and low levels of fluorine with measuredtransmissions of 73.8%/6.4 mm and internal transmittances of 87.9%/cm at157-nm. From extrapolated refractive index measurements of samples, wecalculate a theoretical limit for the measured transmission (reflectionlosses only) of about 88% at 157-nm. The glass has shown to have highresistance to laser-induced color center formation. The thermalexpansion and Young's Modulus of the glass are lower than that ofcommercially available Coming HPFS brand® fused silica glass, whilethermal conductivity is similar. Silicon oxyfluoride glass photomasksubstrates of the invention have been shown to behave similarly tostandard fused silica substrates in mask-making processes such aspolishing and Cr film deposition.

Low OH silica and fluorine-doped, low OH silica samples were prepared bya two-step soot consolidation process. Soot preforms were made by aflame deposition method then dried, fluorine-doped, and sintered in ahigh temperature furnace. High OH fused silica samples were CorningHPFS® which were prepared by a one-step flame hydrolysis process. OHlevels of the glasses described here were quantified by measuring thefundamental OH stretching vibration using infrared spectroscopy.Fluorine levels were measured by microprobe analysis.

The silicon oxyfluoride glass substrates used in the mask processingstudies were cut and polished. Cleaning and Cr film deposition wereperformed in accordance with photomask industry practice.

Transmission data were recorded on an Acton model VTMS-502 VacuumTransmittance Measurement System. The dispersive and detector componentsconsisted of a focused deuterium light source, a single monochromatorwith adjustable entrance and exit slits, and a detector interface thatutilizes a photomultiplier tube. VUV transmittance measurements weremade under vacuum.

For the 157-nm exposures, a TuiLaser ExciStar S200 F₂ laser was used.The energy through the aperture was monitored using a Molectron thermaldetector. Vacuum UV and UV measurements were made after exposure.

Refractive index measurements were made on prisms. Index measurements atvisible wavelengths (643.8-nm, 589.3-nm, 546.1-nm, 480.0-nm, and435.8-nm) were made on a Bausch and Lomb low range refractometer withsodium and Hg-Cd emission lamps. A refractive index standard, certifiedby the National Institute of Standards and Technology (NIST), wasmeasured along with the samples and used to correct the sample readings.Refractive index measurements at near IR wavelengths (777-nm, 1300-nm,and 1541-nm) were made on a Metricon Model 2010 Prism Coupler with laserdiode sources. The accuracy of the visible and near IR measurements isestimated as ±0.0001.

FIG. 1 compares the VUV transmission spectra of Corning HPFS® UV excimergrade fused silica, (Glass A), dry silica containing no fluorine (GlassB), and the inventive silicon oxyfluoride glass dry silica containing0.94 wt. % fluorine (Glass C). Glass A contained about 860 ppm wt. OH,while the OH contents of Glasses B and C were below the detection limitof the measurement (<1 ppm wt.). All of the samples were 5-mm thick anddid not receive any special surface cleaning before the measurement.Glass A does not transmit at 157-nm. In Glass B the UV absorption edgeis shifted to shorter wavelengths and the glass shows some transmissionat 157-nm. In Glass C the UV absorption edge is shifted to even shorterwavelengths and the glass exhibits significant transmission at 157-nm,79%/5 mm.

The internal transmittance at 157-nm of Glass C was determined byplotting the measured optical density at 157-nm vs. thickness for threedifferent pathlengths (FIG. 2). The data were measured on opticallypolished samples with 5 angstrom RA surfaces. The slope of the lineyields the absorption coefficient, which is a measure of the internaltransmittance of the material. The “zero-thickness” intercept of theline is a measure of surface losses due to reflections, surfacescattering, and surface contamination. The internal transmittance of theglass, calculated from the slope of the line, was found to be 87.9%/cm.The measured 157-nm transmission of Glass C, which includes internal andsurface losses, was 73.8%/6.4 mm. From the intercept, it is calculatedthat 20% of the 26.2% measured losses are due to surface losses, whichare highly dependent on sample surface preparation and cleaning, and,therefore, only 6.2% are due to internal loss mechanisms within theglass.

Internal loss mechanisms are scattering and absorption within thematerial. To separate the two mechanisms, we have made scatteringmeasurements on the silicon oxyfluoride glass at wavelengths down to193-nm. Measurements of scattered intensity were made on samples ofsilicon oxyfluoride glass and Corning HPFS® using a 193-nm laser,vertically polarized, at a 90° scattering angle. The ratio of themeasured intensities was near unity. The scattering loss at 193-nm ofCorning HPFS® had previously been determined to be 0.15%/cm. Using a λ⁻⁴extrapolation to 157-nm provides an estimate of about 0.34%/cmscattering loss. This is a very small loss compared to the internallosses of 12.1%/cm measured in Glass C. Therefore, the primary lossmechanism in Glass C is most likely absorption.

FIG. 3 compares the absorption spectra of Glass B and an inventivesilicon oxyfluoride Glass D (0.2 wt. % F) before and after exposure toF₂ radiation. All of the spectra have been normalized to 100%transmission at 400-nm. Glass B was given an exposure of 0.96×10⁶ pulsesat 4 mJ/cm²-pulse. The VUV measurement showed a significant increase inabsorption over the entire wavelength range, 155-220 nm, and formationof the 215-nm band (E′ centers). Glass D was given a much higherexposure of 69×10⁶ pulses at 4 mJ/cm²-pulse, yet the glass exhibited amuch smaller increase in absorption and the 215-nm band is notsubstantially detectable.

FIG. 4 shows refractive index measurements on a silicon oxyfluorideGlass E (0.8 wt. %F) at eight wavelengths in the visible and near IR.The relationship between refractive index and wavelength is generallydescribed by the three-term Sellmeier equation: $\begin{matrix}{{{n(\lambda)}^{2} - 1} = {\sum\limits_{j = {1 - 3}}( \frac{S_{j}\lambda^{2}}{\lambda^{2} - \lambda_{j}^{2}} )}} & (1)\end{matrix}$

where n(λ) is the refractive index at wavelength λ (in μm), and S_(j),λ_(j) are the fitting parameters. Fitting the experimentally determinedindex values to the Sellmeier equation using a least squares fittingsequence, the parameters for Glass E were determined to be: S₁=0.69761,λ₁=0.06630, S₂=0.39778, λ₂=0.11832, S₃=0.88059, and λ₃=9.9118.Extrapolating Equation 1 to 157-nm, the refractive index for Glass E wascalculated as 1.6733 at 157-nm.

It is known that the refractive index of silica is decreased by theaddition of fluorine. To see the effect at 157-nm, we performedrefractive index measurements through the visible and near IR andSellmeier analyses as described above on dry silica glasses containingfrom 0 to 1.5 wt. % F. FIG. 5 shows a plot of refractive indexmeasurements at 435.8-nm as a function of fluorine content. FIG. 6 showsthe plot of calculated 157-nm index as a function of fluorine content.From linear regressions performed on the data in FIGS. 5 and 6, a changein index (100 Δn/n) of −0.30% per 1 wt. % F at 435.8-nm and −0.32% per 1wt. % F at 157-nm was calculated.

We have further characterized the thermal and mechanical properties ofdry fused silica over the same range of fluorine contents. As anexample, FIG. 7 shows the coefficient of thermal expansion (CTE) as afunction of fluorine level. The data were taken on samples that had beenannealed by heating for 1 hour at the annealing point(viscosity=10^(13.2) Poise) and cooling at a rate of 100° C./hour toroom temperature. From the linear fit to the data in FIG. 7, it is foundthat fluorine produces a decrease in CTE of about 0.11 ppm/° C. per 1wt. % F.

Table 1 summarizes the results of our optical and physical propertymeasurements. The table compares properties of Corning HPFS® (Glass A)and silicon oxyfluoride glass (Glass E) and shows the effect of changesin fluorine content on the properties of the glass. Changes inproperties with F content were taken from the linear fits to measuredproperty data similar to FIG. 7.

TABLE 1 Comparison of optical and physical properties of standard UVexcimer grade silica (Glass A) and modified fused silica (Glass E, 0.8wt. % F). Optical properties are for 193 nm (Glass A) and 157 nm (GlassE). (nc = no change_ Change in property Property Glass A Glass E (per 1wt. % F addition Optical: Refractive Index 1.560841 1.6733 −0.0053Thermal: Coefficient of Thermal 0.57 0.51 −0.11 Exapansion, 300° C.-RT(ppm/° C.) Specific Heat, 298K 46 46 nc (J/mol-K) Thermal Conductivity,1.3 1.3 nc 298K (W/m-K) Mechanical: Young's Modulus, MPa 72700 69300−4511 Shear Modulus, Mpa 31400 29600 −2059 Poisson's Ratio 0.16 0.17+0.005

Photomask polishing and coating deposition experiments were performed toreveal any gross differences between the processing of siliconoxyfluoride glass and standard high purity fused silica photomasksubstrates.

Polished substrates, 25 mm×25 mm×1.5 mm thick, were prepared from thesilicon oxyfluoride glass. For comparison, 25 mm×25 mm×6.35 mm thick,substrates were also cut from a standard commercial silica photomasksubstrate. The substrates were cleaned in a sulfuric acid/peroxidesolution and mask detergent, then spin-dried and baked at 120° C. Crfilms, 100 nm thick, were deposited by sputtering.

Film adhesion measurements were made by indentation and scratch testingusing a Nanoindenter II. Under the same test conditions, delamination ofthe Cr film could not be induced on either type of substrate. Theseresults indicate good film adhesion.

Substrates of Corning HPFS® fused silica and of the silicon oxyfluorideglass were polished to 0.2 nm rms surface roughness using chemicalmechanical polishing.

The theoretical transmission limit of the silicon oxyfluoride glass,defined as losses due only to reflections, can be calculated from therefractive index of the material at 157-nm (n) using the formula:$\begin{matrix}{{\% {T( {{theor}.} )}} = {\frac{2n}{n^{2} + 1} \times 100}} & (2)\end{matrix}$

This equation for % T(theor.) is based on multiple internal reflections.From the linear fit to the experimental data in FIG. 6, a 157-nmrefractive index for Glass C (0.94 wt. % F) of 1.6730 was calculated.Using this index in Equation 2, a theoretical transmission limit ofabout 88% at 157-nm is predicted. Since this limit is based on arefractive index calculated from an extrapolation of the Sellmeierequation, it should be considered an approximation based on the bestavailable data.

The measured transmission of Glass C was 73.8% through 6.4 mm.Improvements towards the theoretical limit of 88% could come fromdecreases in surface losses or internal losses. Surface losses arehighly dependent on sample surface preparation and cleaning. Theimportant material property, is the internal transmittance of the glass.The internal transmittance of silicon oxyfluoride Glass C was 87.9%/cm.157-nm is very close to the UV absorption edge of the siliconoxyfluoride where residual impurities (including OH and Cl) and otherdefects in the glass structure have a large effect on the transmittance.

The silica glass structure can be described as a network of SiO₄tetrahedra bonded together at all four corners and randomly orientedwith respect to one another. Water is incorporated into the structure as≡Si—OH (where ≡ indicates bonding to the SiO₄ network) such that thebond to the neighboring tetrahedron is broken at the OH group. OHproduces absorption in the deep UV at <175-nm. Fluorine is similarlyincorporated into the structure as ≡Si—F with the connectivity of thenetwork being broken at the F atom. Electronic transitions associatedwith the Si—F bond are expected to be at higher energies (shorterwavelengths) than those from the Si—O network bonds.

The fluorine-doped silica structure is particularly resistant to damageby F₂ excimer laser irradiation. Here, we have shown that formation ofE′ color centers is highly suppressed in the fluorinated structure, evenin silica containing a very low concentration of fluorine. It ispossible that the fluorine reduces the number of precursor sites forcolor center formation such as weak or strained bonds andoxygen-deficient Si—Si defects.

Chlorine content has been found to drastically effect the 157 nmtransmission of the silicon oxyfluoride glass. A plot of 157 nmabsorption of 5 mm thick silicon oxyfluoride glass samples versus thechlorine concentrations of the samples shows as in FIG. 11, that the 157nm transmission is improved by low levels of chlorine, preferably withthe glass chlorine free.

Our measurements show that silicon oxyfluoride glass has similaroptical, thermal and mechanical properties as standard UV excimer gradesilica. The addition of fluorine to the silica structure does producemeasurable changes in most of these properties. Thermal expansion,Young's modulus, and shear modulus all decrease with fluorine content,while thermal conductivity and specific heat are largely unaffected. Thesmaller thermal expansion of silicon oxyfluoride glass may prove to be abenefit in 157 nm applications and can be utilized in other lithographyand photomask applications such as reflective lithography systems,particularly very short wavelength systems. The refractive index of theglass is also decreased by fluorine. However, because of the indexdispersion, the refractive index of silicon oxyfluoride glass isconsiderably higher at 157 nm than standard excimer grade fused silicaat 193 nm. In comparison, CaF₂ has a substantially higher thermalconductivity and lower 157 nm refractive index than silica both of whichare beneficial to the photomask application, but its high thermalexpansion coefficient and crystalline nature make fabrication and maskprocessing very difficult.

Silicon oxyfluoride with internal transmittances as high as 87.9%/cmhave been prepared. Scattering measurements indicate that scatteringlosses at 157-nm are very low so that the primary loss mechanism in theglass is absorption. From refractive index measurements, the theoreticallimit for measured transmission (reflection losses only) is predicted tobe around 88% at 157 nm. The optical, thermal and mechanical propertiesof silicon oxyfluoride glass are different from those of standard UVexcimer grade fused silica, due to the addition of fluorine to theglass.

In a preferred embodiment the invention includes a below 193 nm VUVtransmitting glass photomask substrate for photolithography atwavelengths of about 157 nm. The VUV transmitting glass substratecomprises high purity oxyfluoride glass with an OH content below 50 ppmby weight, hydrogen content below 1×10¹⁷ molecules cm³, and a fluorinecontent in the range of 0.1 to 0.4 weight percent. Preferably the glasshas a Cl content below 5 ppm, more preferably below 1 ppm, and mostpreferably with the glass being essentially free of chlorine. Preferablythe glass has a molecular hydrogen content below 3×10¹⁶ molecules/cm³,and more preferably has no detectable molecular hydrogen content.Preferably the glass has an OH content below 10 ppm by weight, morepreferably below 1 ppm by weight, and most preferably has no detectableOH content and is essentially OH free. Preferably the glass is comprisedof Si, O, and F and is essentially free of OH, Cl and H₂. Preferably thesilicon oxyfluoride glass photomask substrate has an internaltransmittance at 157 nm of at least 89%/cm, most preferably with thesubstrate having a measured transmittance of at least 79% through athickness of the photomask substrate with the substrate being about 6 mmthick, such as 6.35 mm thick.

In a preferred embodiment the invention includes a process of making aVUV transmitting glass having high resistance to optical damage toexcimer laser radiation in the 157 nm wavelength region. The processincludes providing a plurality of SiO₂ particles and dehydrating theparticles. The process includes fluorine doping and consolidating theparticles to form a dry, non-porous monolithic body of transparent fusedsilicon oxyfluoride glass with a fluorine content less than 0.5 weightpercent.

Preferably the particles are reacted with a fluorine-containing gas suchthat the amount of fluorine incorporated into the glass resulting fromconsolidation is in the range of 0.1 to 0.4 weight percent. Preferablythe fluorine-containing gas is selected from the group consisting ofCF₄, SiF₄, F₂, SF₆, C₂F₆, C₃F₈ and mixtures thereof. In a preferredembodiment the particles are doped with the fluorine-containing gas at adoping temperature in the range of 750° C. to 1050° C., more preferablyin the range of 800° C. to 1000° C., and most preferably at about 800°C. (±25° C.). The preferred fluorine-containing gas for these preferreddoping temperatures is SiF₄ A preferred fluorine doping furnacetreatment atmosphere for these temperatures is a mixture of SiF₄, O₂ andhelium, most preferably with O₂ being present during the fluorine dopingexposure. A preferred fluorine doping furnace treatment atmosphere isabout 5% SiF (±2%, more preferably ±1%) with about 80% O₂ (±10%, morepreferably ±5%) with the remainder being helium. After the fluorinedoping exposure the doped particles are consolidated intosilconoxyfluoride glass in a consolidation furnace sintering zone with asintering temperature in the range of 1400° C. to 1550° C. Preferablythe particles are sintered and consolidated in a cosolidatin furnaceatmosphere of pure helium, such as by down feeding the soot particleblank from a furnace doping zone in to the sintering zone after thesupply of fluorine-containing SiF₄ gas and O₂ is terminated.

The invention includes a F₂ laser-induced absorption resistant siliconoxyfluoride glass suitable for use in the 157 nm wavelength region, theglass having a stable and high transmission at 157.6 nm with a fluorinecontent less than 0.5 weight % such that the glass has a transmissionloss at 157.6 nm<1% after exposure to a F₂ excimer laser for 60 millionpulses at 0.1 mJ/cm²-pulse. The silicon oxyfluoride glass preferably isessentially free of OH groups, has less than 5×10¹⁶ molecules/cm³ ofmolecular hydrogen, and a fluorine content in the range of about 0.1 to0.4 weight %.

The invention includes a F₂ laser-induced absorption resistantlithography glass comprising a silicon oxyfluoride glass having an OHcontent less than 5 ppm, by weight, a Cl content less than 5 ppm byweight, and a fluorine content of 0.1 to 0.4 weight % with a 157 nminternal transmission of at least 80%/cm, more preferably 85%/cm.Preferably the glass has a H2 content less than 1×10¹⁷ molecules/cm³.The silicon oxyfluoride glass is resistant to laser-induced absorptionand has a 157 nm transmission loss <1% after exposure to a 157 nm laserfor 60 million pulses at 0.1 mJ/cm²-pulse. The silicon oxyfluoride glasshas a resistance to 157.6 nm induced absorption, with the fluorinecontent inhibiting 165 nm absorption oxygen-deficient centers.Preferably the glass has a 165 nm absorption less than 0.4 (absorptionunits/5 mm) after exposure to a 157 nm laser for 41.5 million pulses at2 mJ/cm²-pulse of 157 nm laser, and most preferably the 165 nmabsorption is less than 0.2 (absorption units/5 mm). Preferably the Clcontent is less than 1 ppm and the OH content is less than 1 ppm,preferably with the glass consisting essentially of Si, O and F. In anembodiment the glass is a VUV transmitting photomask. In an embodimentthe glass is a VUV phase shifting photomask. Preferably the glassphotomasks have a resistance to laser induced oxygen-deficient centers,preferably with the glass essentially free of metal to metal Si—Si bondsand free of a 165 nm absorbing center and has an internal transmissionat 165 run of at least 85%/cm.

The invention includes a VUV pattern printing method. The patternprinting method includes providing a below 164 nm radiation source forproducing VUV photons, providing a silicon oxyfluoride glass having lessthan 5 ppm by weight OH, less than 5 ppm by weight Cl, a less than 0.5weight percent fluorine content, and a 157 nm and 165 nm measuredtransmission of at least 75%/5 mm. The method includes transmitting theVUV photons through the silicon oxyfluoride glass, forming a patternwith the VUV photons, and projecting the pattern onto a VUV radiationsensitive printing medium to form a printed pattern. Embodiments ofpattern printing methods of the invention are shown in FIGS. 8-9. In apreferred embodiment the VUV lithography pattern printing method whichincludes providing a VUV lithography photon radiation source, providinga silicon oxyfluoride lithography glass with less than 1 ppm OH,transmitting the VUV lithography photons through the silicon oxyfluoridelithography glass, forming a lithography pattern with the VUV photons,and projecting the lithography pattern onto a VUV radiation sensitivelithography printing medium to form a printed lithography pattern.

The VUV pattern printing method preferably includes lowering the VUV cutoff wavelength of the silicon oxyfluoride glass by providing an SiO₂glass forming precursor and doping with an F content to provide asilicon oxyfluoride glass with a 50% transmission VUV cut off wavelengthbelow 160 nm and a 165 nm absorption less than 0.4 (absorption units/5mm) after exposure to a 157 nm laser for 4.5 million pulses at 2mJ/cm²-pulse.

The invention provides superior transmission in the VUV with the siliconoxyfluoride glasses having high purity and being dry (<1 ppm OH). Theinventive glasses have provided 157 nm transmissions such as 79.8%/6.35mm (and 90%/cm internal T). The silicon oxyfluoride glass is preferablyutilized as components for 157 nm lithography, particularly photomasksubstrates, pellicles, thin lenses and windows. For these lithographyapplications, it is not only important that the glass exhibits a highinitial transmission, but the transmission must not decrease underexposure to the F₂ excimer laser. The glass provides a <1% transmissionloss at 157.6 nm after exposure to the F₂ excimer laser for 60 millionpulses at 0.1 mJ/cm²-pulse. Our preferred silicon oxyfluoride glasscompositions exhibit improved resistance to F₂ laser-induced absorption.

We have discovered that although the addition of fluorine to a drysilica glass improves its transmission in the vacuum ultraviolet,particularly near the UV edge (e.g., at 157 nm), high concentrations offluorine in the glass are detrimental to its laser damage resistance.Specifically, we have found that under F₂ excimer laser exposure, theinduced absorption in the glass is proportional to the fluorine contentof the glass. As the fluorine content increases, the 157.6 nm inducedabsorption increases. FIG. 12 is a plot of 157.6nm induced absorptionvs. exposure time for three silicon fluoride glasses with differentfluorine contents. Clearly the magnitude of the induced absorptionincreases with fluorine content. In order to understand what defectcenters are contributing to the induced absorption, we measured theabsorption spectra of the glasses before and after exposure (see FIG.13). The plots in FIG. 13 are for the glasses with the highest andlowest fluorine levels in FIG. 12 and for a dry silica glass containingno fluorine. Three defect centers were identified in the exposedglasses: non-bridging oxygen hole centers (260 nm) (NBOHC), E′ centers(215 nm), and oxygen-deficient centers (165 nm) (ODC). Notice inparticular that the concentration of oxygen-deficient centers formedunder the F₂ laser exposure is dependent on the fluorine concentration.Note that the low fluorine silicon oxyfluoride glass of FIG. 13b (0.12wt. % F) has good laser durability and is resistant to F₂ laser inducedabsorption and free of laser induced oxygen-deficient centers at 165 nmas compared to FIG. 13a. We have seen a similar effect under ArF (193nm) excimer laser exposure, although the dependence on fluorineconcentration does not appear to be nearly as strong as under the F₂excimer laser (see FIG. 14). Finally, we exposed a high fluorine silica(1.7 wt. % F) and a low fluorine silica (0.12 wt. % F) to the F₂ laserunder a low fluence condition to simulate the use condition for a 157 nmphotomask substrate. FIG. 15 shows a plot of 157.6 nm transmission vs.exposure time at 0.2 mJ/cm²-pulse. The low fluorine glass retained itshigh initial transmission (73.5%/6.35 mm) over 41.5 million pulseswhereas the high fluorine glass showed significant loss of transmissionafter just 10 million pulses.

The 1.7 wt. % F glass of FIGS. 12, 13 a, 14 a, and 15 was made from asilica soot preform. The silica soot preform, was prepared using SiCl₄as the source compound. The soot blank was held in a furnace for 60minutes at 1100° C. under a Cl₂/He atmosphere (0.066 slpm Cl₂, 20.64slpm He). The temperature was ramped to 1225° C. over 45 mins, and thesoot preform was exposed to a CF₄/He atmosphere (4 slpm CF₄, 12 slpm He)for 180 minutes. The preform was driven into the high temperature (maxT=1480° C.) part of the furnace under the same gas flows to produce afully dense, F—SiO₂ glass. The fluorine concentration in the glassdetermined by microprobe analysis was 1.7 wt. % F and the chlorineconcentration was 14 ppm wt.

A 1.2 wt. % F glass was made from a silica soot preform. The silica sootpreform, was prepared using SiCl₄ as the source compound. The soot blankwas held in a furnace for 180 minutes at 1000° C. under a Cl₂/Heatmosphere (0.066 slpm Cl₂, 20.64 slpm He). The temperature was rampedto 1225° C. over 45 mins, and the soot preform was exposed to a CF₄/Heatmosphere (1.6 slpm CF₄, 18.4 slpm He) for 120 minutes. The preform wasdriven into the high temperature (max T=1480° C.) part of the furnaceunder the same gas flows to produce a fully dense, F—SiO₂ glass. Thefluorine concentration in the glass determined by microprobe analysiswas 1.2 wt. % F.

The 0.94 wt. % F glass of FIG. 12 was made from a silica soot preform.The silica soot preform, 1532 g, was prepared using SiCl₄ as the sourcecompound. The soot blank was held in a furnace for 60 minutes at 1100°C. under a Cl₂/He atmosphere (0.066 slpm Cl₂, 40.64 slpm He). Thetemperature was ramped to 1225° C. over 45 mins, and the soot preformwas exposed to a CF₄/He atmosphere (0.8 slpm CF₄, 39.2 slpm He) for 150minutes. The preform was driven into the high temperature (max T=1480°C.) part of the furnace under the same gas flows to produce a fullydense, F—SiO₂ glass. The fluorine concentration in the glass determinedby microprobe analysis was 0.94 wt. % F.

The 0.12 wt. % F glass of FIGS. 12, 13 b, 14 b, and 15 was made from asilica soot preform. The silica soot preform, was prepared using OMCTSas the source compound. The soot blank was held in a furnace for 120minutes at 1000° C. under a CF₄/He atmosphere (0.4 slpm CF₄, 40 slpmHe). The preform was then driven into the high temperature (max T=1480°C.) part of the furnace under a pure He flow to produce a fully dense,F—SiO₂ glass. The fluorine concentration in the glass determined bymicroprobe analysis was 0.12 wt. % F.

The dry No Fluorine glass of FIGS. 13c and 14 c was made from a silicasoot preform. The silica soot preform was made using OMCTS as the sourcecompound. The soot preform was dried at 1100-1150° C. and sintered to afully dense glass under a Cl₂/He atmosphere. Microprobe analysis showedthe glass to contain no fluorine and 1290 ppm wt of chlorine.

A 0.53 wt. % F glass was made from a silica soot preform. The silicasoot preform, 892 g, was prepared using OMCTS as the source compound.The soot blank was held in a furnace for 240 minutes at 1000° C. under aSiF₄/O₂/He atmosphere (1 slpm SiF₄, 16 slpm O2, 3 slpm He). The preformwas then driven into the high temperature (max T=1480° C.) part of thefurnace under a pure He flow to produce a fully dense, F—SiO₂ glass. Thefluorine concentration in the glass determined by microprobe analysiswas 0.53 wt. % F and the chlorine concentration was 19 ppm wt.

A 0.31 wt. % F glass was made from a silica soot preform. The silicasoot preform, was prepared using OMCTS as the source compound. The sootblank was held in a furnace for 240 minutes at 800° C. under aSiF₄/O₂/He atmosphere (1 slpm SiF₄, 16 slpm O₂, 3 slpm He). The preformwas then driven into the high temperature (max T=1480° C.) part of thefurnace under a pure He flow to produce a fully dense, F—SiO₂ glass. Thefluorine concentration in the glass determined by microprobe analysiswas 0.31 wt. %F.

A 0.26 wt. % F glass was made from a silica soot preform. The silicasoot preform, was prepared using OMCTS as the source compound. The sootblank was held in a furnace for 240 minutes at 800° C. under aSiF₄/O₂/He atmosphere (0.6 slpm SiF₄, 16 slpm O₂, 3.4 slpm He). Thepreform was then driven into the high temperature (max T=1480° C.) partof the furnace under a pure He flow to produce a fully dense, F—SiO₂glass. The fluorine concentration in the glass determined by microprobeanalysis was 0.26 wt. % F and the chlorine concentration was 15 ppm wt.

A 0.17 wt. % F glass was made from a silica soot preform. The silicasoot preform, was prepared using SiCl₄ as the source compound. The sootblank was held in a furnace for 60 minutes at 1000° C. under a Cl₂/Heatmosphere (0.066 slpm Cl₂, 40.64 slpm He). The temperature was rampedto 1100° C. over 20 mins, and the soot preform was exposed to a CF₄/Heatmosphere (0.4 slpm CF₄, 40 slpm He) for 120 minutes. The preform wasdriven into the high temperature (max T=1480° C.) part of the furnaceunder a pure He flow to produce a fully dense, F—SiO₂ glass. Thefluorine concentration in the glass determined by microprobe analysiswas 0.17 wt. %.

A 0.05 wt. % F glass was made from a silica soot preform. The silicasoot preform, was prepared using OMCTS as the source compound. The sootblank was held in a furnace for 120 minutes at 800° C. under aSiF₄/O₂/He atmosphere (1 slpm SiF₄, 4 slpm O₂, 15 slpm He). The SiF₄ andO₂ were turned off and the preform was held at 800° C. under a pure Heatmosphere for 120 minutes. The preform was then driven into the hightemperature (max T=1480° C.) part of the furnace under a pure He flow toproduce a fully dense, F—SiO₂ glass. The fluorine concentration in theglass determined by microprobe analysis was 0.05 wt. % F.

Preferably our silicon oxyfluoride glass has a fluoride content lessthan 0.5 wt. % F. Our <0.5 wt. % F dry silicon oxyfluoride glassexhibits good laser durability and suitability for applicationsinvolving exposure to F₂ excimer laser radiation. Our preferredcomposition range for attaining both high initial transmission and lowinduced absorption is 0.1-0.4 wt. % F.

The invention includes a VUV transmitting silicon oxyfluoride glasshaving an OH content less than 5 ppm by weight, a fluorine content of atleast 0.1 weight %. Preferably the glass consists essentially of Si, Oand F with the glass having an internal transmission in the wavelengthrange of 157 nm to 175 nm of at least 85%/cm. Preferably the glass has a165 nm absorption less than 0.4 (absorption units/5 mm) after exposureto a 157 nm laser for 41.5 million pulses at 2 mJ/cm²-pulse.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present inventionwithout departing from the spirit and scope of the invention. Thus, itis intended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A below 193 nm VUV transmitting glass photomasksubstrate for photolithography at wavelengths of about 157 nm comprisinghigh purity silicon oxyfluoride glass with an OH content below 50 ppm byweight, hydrogen content below 1×10¹⁷ molecules/cm³, and fluorinecontent in the range of 0.1 to 0.4 weight percent.
 2. A VUV photomasksubstrate according to claim 1 wherein the glass has a Cl content below5 ppm.
 3. A VUV photomask substrate according to claim 1 wherein themolecular hydrogen content is below 3×10¹⁶ molecules/cm³.
 4. A VUVphotomask substrate according to claim 1 wherein the fused silica ischaracterized by having no detectable molecular hydrogen content.
 5. AVUV photomask substrate according to claim 1 wherein the OH content isbelow 10 ppm by weight.
 6. A VUV photomask substrate according to claim1 wherein the OH content below 1 ppm by weight.
 7. A VUV photomasksubstrate according to claim 1 further characterized by having internaltransmittance of at least 89%/cm at 157 nm wavelength region.
 8. A VUVphotomask substrate according to claim 7 wherein the measuredtransmittance is at least 79% through a thickness of the photomasksubstrate.
 9. A VUV photomask substrate according to claim 8, whereinthe thickness is about 6 mm.
 10. A VUV photomask substrate according toclaim 1, further characterized by being essentially free of chlorine.11. A VUV photomask substrate according to claim 1, said siliconoxyfluoride glass comprised of Si, O and F and is essentially free ofOH, Cl and H₂.
 12. A VUV photomask substrate according to claim 1,wherein said glass has a low H₂ content such that less than 10¹⁸ H₂molecules/m² are released when heated under a vacuum to about 1000° C.13. A VUV photomask substrate according to claim 1, wherein saidphotomask substrate is free of an absorption peak at 4,100 cm⁻¹.
 14. Asilicon oxyfluoride glass suitable for use in the 157 nm wavelengthregion, said glass having essentially no OH groups, 0.1 to 0.4 weight %fluorine, and less than 5×10¹⁶ molecules/cm³ of molecular hydrogen. 15.A lithography glass comprising a silicon oxyfluoride glass, said siliconoxyfluoride glass having an OH content less than 5 ppm by weight, a Clcontent less than 5 ppm by weight, a H₂ content less than 1×10¹⁷molecules/cm³, and a fluorine content of 0.1 to 0.4 weight %, said glasshaving a 157 nm internal transmission of at least 85%/cm.
 16. Alithography glass as claimed in claim 15, wherein said glass has a 157nm transmission loss <1% after exposure to a 157 nm laser for 60 millionpulses at 0.1 mJ/cm²-pulse.
 17. A lithography glass as claimed in claim15, wherein said glass has a resistance to 157.6 nm induced absorption.18. A lithography glass as claimed in claim 15, wherein said fluorinecontent inhibits 165 nm absorption oxygen-deficient centers.
 19. Alithography glass as claimed in claim 15, wherein said glass has a 165nm absorption less than 0.4 (absorption units/5 mm) after exposure to a157 nm laser for 41.5 million pulses at 2 mJ/cm² pulse.
 20. Alithography glass as claimed in claim 19, said 165 nm absorption lessthan 0.3 (absorption units/5 mm) after a 157 nm laser for 41.5 millionpulses at 2 mJ/cm² pulse.
 21. A glass as claimed in claim 20 whereinsaid 165 nm absorption is less than 0.2 (absorption units/5 mm).
 22. Aglass as claimed in claim 15 wherein said Cl content is less than 1 ppmand said OH content is less than 1 ppm.
 23. A glass as claimed in claim15 wherein said glass consists essentially of Si, O, and F.
 24. A VUVtransmitting photomask comprised of the glass as claimed in claim 15.25. A VUV phase shifting photomask comprised of the glass as claimed inclaim
 15. 26. A photomask of the glass as claimed in claim 15, saidphotomask having a resistance to laser induced oxygen deficient centers.27. A glass as claimed in claim 15 wherein said glass is essentiallyfree of metal to metal Si—Si bonds.
 28. A photomask as claimed in claim27 wherein said glass is free of a 165 nm absorbing center and has aninternal transmission at 165 nm of at least 85%/cm.
 29. A VUV patternprinting method, said method comprising: providing a below 164 nmradiation source for producing VUV photons, providing a siliconoxyfluoride glass having less than 5 ppm by weight OH, less than 5 ppmby weight Cl and a <0.5 weight percent fluorine content, and 157 nm and165 nm measured transmission of at least 75%/5 mm, transmitting said VUVphotons through said silicon oxyfluoride glass, forming a pattern withsaid VUV photons, and projecting said pattern onto a VUV radiationsensitive printing medium to form a printed lithography pattern.
 30. Amethod as claimed in claim 29 wherein providing a silicon oxyfluorideglass comprises lowering the VUV cut off wavelength of the glass byproviding an SiO₂ glass forming precursor, and doping with a F contentto provide a silicon oxyfluoride glass with a 50% transmission VUV cutoff wavelength below 160 nm and a 165 nm absorption less than 0.4(absorption units/5 mm) after exposure to a 157 nm laser for 41.5million pulses at 2 mJ/cm²-pulse.
 31. A method as claimed in claim 29wherein said silicon oxyfluoride glass consists essentially of Si, O,and F and is essentially free of Si—Si bonds.
 32. A VUV transmittingsilicon oxyfluoride glass having an OH content less than 1 ppm byweight, a Cl content less than 1 ppm by weight, a H₂ content less than1×10¹⁷ molecules/cm³, a fluorine content of 0.1 to 0.4 weight %, saidglass having a 165 nm absorption less than 0.4 (absorption units/5 mm)after exposure to a 157 nm laser for 41.5 million pulses at 2mJ/cm²-pulse.
 33. A VUV transmitting silicon oxyfluoride glass having anOH content less than 5 ppm by weight, a Cl content less than 5 ppm byweight, a H₂ content less than 1×10¹⁷ molecules/cm³, a fluorine contentof at least 0.1 weight %, said glass consisting essentially of Si, O andF and having a 165 nm absorption less than 0.4 (absorption units/5 mm)after exposure to a 157 nm laser for 41.5 million pulses at 2mJ/cm²-pulse.